Spin-Charge Conversion in an Epitaxial Kagome Flat-Band Metal

The formation of topological flat bands near the Fermi level in kagome-structured metals has generated substantial research interest. One key consequence of this electronic structure is the strong anisotropy in charge transport, such as the significantly higher in-plane resistivity relative to the out-of-plane direction. However, the spin transport properties of these flat-band kagome metals are still largely unexplored. In this work, we investigate spin-charge conversion in CoSn, a representative flat-band kagome metal. Scanning tunneling microscopy is employed to image the CoSn surface with atomic resolution, and $dI/dV$ spectroscopy shows clear signatures of the flat bands. Using magneto-optical techniques, we quantify the spin-orbit torque (SOT) in bilayers of ferromagnetic nickel (Ni) and c-plane CoSn thin films. We find a large effective spin Hall angle of $θ_{SH} = 0.484 \pm 0.005$ within the CoSn layers, significantly exceeding the value measured for Pt of $θ_{SH} = 0.0436 \pm 0.0003$ in Ni/Pt bilayers. Density functional theory (DFT) calculations provide a microscopic understanding of the intrinsic spin Hall effect in CoSn, elucidating the sources of Berry curvature in momentum space. These findings establish kagome metals as promising materials for efficient spin-charge conversion and low-power spintronics.